Node.js 节点js POST请求错误[ERR_HTTP_HEADERS_SENT]：发送到客户端后无法设置头

我已经使用NodeJS、Express和MongoDB开发了REST服务。 我已经定义了一个POST请求，将用户添加到DB表中，当我尝试（在本地主机上）POSTMAN提供的REST服务时，他会工作并将用户添加到表中，但节点应用程序崩溃（server.js），我收到以下错误： 抛出新的ERR_HTTP_头_SENT（'set'）； ^

错误[ERR\u HTTP\u HEADERS\u SENT]：发送到客户端后无法设置头 此错误与我的代码有关：

router.post('/user', VerifyToken, function (req, res) {
User.findOne({ username: req.body.username }, function (err, user) {
    if (user) return res.status(400).send({status: 'ko', error: { msg: 'The username you have entered is already associated with another user.'}});

    var hashedPassword = bcrypt.hashSync(req.body.password, 8);
    User.create({
        firstname:req.body.firstname,
        surname:req.body.surname,
        username:req.body.username,
        email: req.body.email,
        password: hashedPassword,
        farmId: req.body.farmId,
        roles: req.body.roles,
        isVerified : req.body.isVerified,
        statusUser : req.body.statusUser
        },
        function (err, user) {
            if (err) return res.status(500).send("There was a problem adding the user to the database.");
            res.status(200).send({status: 'ok', data: { msg: 'User saved', user: user}});

            var client = nodemailer.createTransport(sgTransport(options));

            var email = {
                from: 'noreply-tlcplus@trelleborg.com',
                to: req.body.email,
                subject: 'Registration successfully confirmed',
                text: 'Hi '+ req.body.username + ',\n\nyour account has been registered.\n\nAre you the farm owner?' +
                '\n\nPlease go to this link [CMS link] to create your Profile and start to use the App Plus!\n\n'+
                'If you are a simple user, please just use your credentials to login to the App Plus section into the new App!\n\n'+
                'Download for free from App Store or Google Play!\n\nRegards,\n\nTrelleborg TLC Plus team'
            };

            client.sendMail(email, function(err, json){
                if (err){
                    return res.status(500).send({ msg: err.message });
                }
                else {
                    res.status(200).send({status: 'ok', data: { msg: 'A verification email has been sent to ' + user.email + '.'}} )
                }
            });

        });

});

});

行错误位于res.status（200）上，用于发送验证邮件。 为什么我的应用程序崩溃，错误在哪里？ 请问有什么帮助吗？

---

谢谢

您不能为一个请求发送多个响应。您向请求发送了两次响应。一个是在创建记录之后，另一个是在发送邮件之后。创建用户后发送，并在后台发送电子邮件，或者仅在邮件发送过程完成后发送成功响应

---

希望这有帮助：）

发生这种情况是因为您发送了两次响应。因此，错误是：

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

将代码更改为以下内容：

router.post('/user', VerifyToken, function (req, res) {
User.findOne({ username: req.body.username }, function (err, user) {
    if (user) return res.status(400).send({status: 'ko', error: { msg: 'The username you have entered is already associated with another user.'}});

    var hashedPassword = bcrypt.hashSync(req.body.password, 8);
    User.create({
        firstname:req.body.firstname,
        surname:req.body.surname,
        username:req.body.username,
        email: req.body.email,
        password: hashedPassword,
        farmId: req.body.farmId,
        roles: req.body.roles,
        isVerified : req.body.isVerified,
        statusUser : req.body.statusUser
        },
        function (err, user) {
            if (err) return res.status(500).send("There was a problem adding the user to the database.");


            var client = nodemailer.createTransport(sgTransport(options));

            var email = {
                from: 'noreply-tlcplus@trelleborg.com',
                to: req.body.email,
                subject: 'Registration successfully confirmed',
                text: 'Hi '+ req.body.username + ',\n\nyour account has been registered.\n\nAre you the farm owner?' +
                '\n\nPlease go to this link [CMS link] to create your Profile and start to use the App Plus!\n\n'+
                'If you are a simple user, please just use your credentials to login to the App Plus section into the new App!\n\n'+
                'Download for free from App Store or Google Play!\n\nRegards,\n\nTrelleborg TLC Plus team'
            };

            client.sendMail(email, function(err, json){
                if (err){
                    return res.status(500).send({ msg: err.message });
                }

               res.status(200).send({status: 'ok', data: { msg: 'A verification email has been sent to ' + user.email + '.'}, message: 'User saved.'} )

            });

        });

});

});